We are studying the effects of diethylstilbesterol on phosphatidylcholine biosynthesis in rooster liver as a model system for hormonal control of phospholipid biosynthesis. Injection of this steroid (20 mg/kg body weight) results in a large increase in serum phospholipids and an enhanced incorporation of 3H-choline into phosphatidylcholine. The studies should pinpoint which reaction in phosphatidylcholine biosynthesis is stimulated by hormone action. Subsequent studies will be directed to how the hormone causes the reaction to be stimulated. Current evidence suggests that the reaction catalyzed by CTP:phosphocholine cytidylyltransferase is stimulated by the hormone treatment. This is caused by a stimulation of choline kinase activity which results in an increase in the concentration of phosphocholine a substrate for the cytidylyltransferase reaction. The results should lead to a better understanding of the control of phosphatidylcholine biosynthesis and the secretion of phospholipids and lipoproteins by liver.